The ‘algebra of evolution’: the Robertson-Price identity and viability selection for body mass in a wild bird population

By the Robertson-Price Identity, the change in a quantitative trait due to selection is equal to the trait’s covariance with relative fitness. We used a long-term study of superb fairy-wrens Malurus cyaneus to consider phenotypic and genetic change within a generation due to juvenile viability selection. Mortality in the four-week period between fledging and independence was 40%, and heavier nestlings were more likely to survive, but why? There was additive genetic variance for both nestling mass and survival, and a positive phenotypic covariance between the two traits, but no evidence of additive genetic covariance. Comparing standardised gradients, the phenotypic selection gradient was positive, βP =0.108 (0.036, 0.187 95%CI), whereas the genetic gradient was not different from zero, βA = ‑0.025 (‑0.19, 0.107 95%CI). This suggests that factors other than nestling mass were the cause of variation in survival. In particular, there were temporal correlations between mass and survival bot...